Stimulus control and compounding with ambient odor as a discriminative stimulus on a free-operant baseline

Abstract

Previous experiments have demonstrated that the simultaneous presentation of independently established discriminative stimuli can control rates of operant responding substantially higher than the rates occasioned by the individual stimuli. This "additive summation" phenomenon has been shown with a variety of different reinforcers (e.g., food, water, shock avoidance, cocaine, and heroin). Discriminative stimuli previously used in such studies have been limited to the visual and auditory sensory modalities. The present experiment sought to (1) establish stimulus control on a free-operant baseline with an ambient olfactory discriminative stimulus, (2) compare olfactory control to that produced with an auditory discriminative stimulus, and (3) determine whether compounding independently established olfactory and auditory discriminative stimuli produces additive summation. Rats lever pressed for food on a variable-interval schedule in the presence of either a tone or an odor, with comparable control developed to each stimulus. In the absence of these stimuli responding was not reinforced. During stimulus compounding tests, the tone-plus-odor compound occasioned more than double the responses occasioned by either the tone or odor presented individually. Thus, the current study (1) established stimulus control with an ambient olfactory discriminative stimulus in a traditional free-operant setting and (2) extended the generality of stimulus-compounding effects by demonstrating additive summation when olfactory and auditory discriminative stimuli were presented simultaneously.

Fig 1. The operant chamber contained within the sound-attenuation chest.

The Tygon tubing (D) used to deliver odorant into the air stream can be seen suspended between the sound-attenuation chest and the 10-cm dryer tubing (F) attached to the top of the operant chamber. See the Apparatus section for a description of the ambient odor presentation, maintenance, and elimination.

Fig 2. Representative 30-min segments of cumulative records of Rats O-1, O-2, O-3, and O-4 from a terminal criterion baseline training session on the three-ply multiple VI VI EXT schedule.

For Rat O-1 the record contains data collected 3 days prior to testing (session 220) For Rat O-2 the record contains data collected on session 136, 2 days prior to testing. For Rats O-3 and O-4, the record contains data collected on the last day of terminal baseline training (session 232 and 194, respectively). Tone (T) or odor (O) components are indicated by corresponding letters above the record. When the pen is depressed neither stimulus was present, and responding was on extinction. The slash marks by the response pen denote the delivery of a reinforcer.

Fig 3. The distribution of tone, odor, and tone-plus-odor (T+O) responses on the stimulus compounding test.

Percentages were calculated for each subject by dividing the responses emitted to each test condition by the total responses emitted to T, O, and T+O summed over the test, and then multiplying by 100. Mean test percentages are presented to the right.

Fig 4. The upper four graphs present the total test responses emitted by individual rats accumulated over each block-randomized presentation of Tone, Odor, and Tone+Odor.

Mean cumulative response data are presented in the lower left graph. The lower right graph presents the mean data in terms of mean cumulative response percentages that weights all animals comparably irrespective of their absolute response rate (see text for details of how these percentages were calculated).